1. Technical Field
The present teaching relates to method and system for light emitting diodes (LED). More specifically, the present teaching relates to method and system for LED dimming and systems incorporating the same.
2. Discussion of Technical Background
LED lighting has been widely utilized in different application scenarios. To save energy and cost, dimming technologies have also been developed so that the lighting can be dimmed in different situations. Traditionally, there are different categories of dimming methods, including pulsed width modulation (PWM) dimming and analog dimming. In PWM dimming, the amount of the LED current used for driving the LED light is usually determined based on the pulse width and period of a PWM signal while in analog dimming, the amount of LED current used to drive the LED light is conventionally determined based on the amplitude of an analog signal. In some applications, PWM dimming and analog dimming can be applied to control the LED current but as separate optional choices. That is, one pin of the LED dimming control may be used to supply a PWM signal for PWM dimming control and another pin may be separately provided so that an analog signal may be individually supplied for analog dimming purposes. A user may be provided with a means to select one or the other approach to control the LED dimming. Although the user has a choice of either dimming approach, traditionally at any given time, only one method is elected so that the other pin may not be utilized. This makes inefficient use of pins.
There are other disadvantages associated with the traditional LED dimming based on PWM dimming. To improve the dimming range of PWM dimming, a common solution is to push the PWM pulse width to reach a lowest level possible. However, when the PWM dimming pulse width is less than a threshold minimum pulse width, various problems may arise. Although such a threshold pulse width is often disclosed in a datasheet associated with a product, customers often exceed this lower minimum making the performance of the product unpredictable. For example, when the pulse width is lower than the specified minimum value, the output LED current and voltage may collapse completely. If this situation occurs, depending on the design, it sometimes requires the next pulse width to be extra long to jump start the circuit to bring back the output.
In addition, when power is turned on with PWM pulses having pulse width smaller than the specified minimum width, certain fault detection and protection features may not work due to the blanking time in some integrated circuits. Furthermore, when the pulse width is smaller than the minimum requirement, the actual peak LED current often will not reach the programmed level, failing to deliver the desired dimming effect. To make it worse, when PWM dimming is operating at a high temperature condition, due to leakage, the PWM dimming ratio often reduces so that the highest PWM dimming range as specified for the product can not be achieved without using lower leakage components. Therefore, a need exists to have an improved PWM dimming approach to solve those problems.